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Potassium capture by coal fly ash K2CO3, KCl and K2SO4
Wang, Guoliang; Jensen, Peter Arendt; Wu, Hao; Frandsen, Flemming Jappe; Laxminarayan, Yashasvi;Sander, Bo; Glarborg, Peter
Published in:Fuel Processing Technology
Link to article, DOI:10.1016/j.fuproc.2019.05.038
Publication date:2019
Document VersionPeer reviewed version
Link back to DTU Orbit
Citation (APA):Wang, G., Jensen, P. A., Wu, H., Frandsen, F. J., Laxminarayan, Y., Sander, B., & Glarborg, P. (2019).Potassium capture by coal fly ash K
2CO
3, KCl and K
2SO
4. Fuel Processing Technology, 194, [106115].
https://doi.org/10.1016/j.fuproc.2019.05.038
1
Potassium Capture by Coal Fly Ash: K2CO3, KCl
and K2SO4
Guoliang Wang a*
, Peter Arendt Jensen a, Hao Wu
a, Flemming Jappe Frandsen
a, Yashasvi
Laxminarayan a, Bo Sander
b, Peter Glarborg
a
a Department of Chemical and Biochemical Engineering, Technical University of Denmark,
Søltofts Plads, Building 229, DK-2800 Kgs. Lyngby, Denmark
b Ørsted Bioenergy & Thermal Power A/S, Kraftværksvej 53, 7000 Fredericia, Denmark
*Corresponding Author E-mail: [email protected]
Permanent address:
Department of Chemical and Biochemical Engineering, Technical University of Denmark,
Søltofts Plads, Building 229, DK-2800 Kgs. Lyngby, Denmark
Manuscript submitted to Fuel Processing Technology
2
Abstract: 1
The potassium capture behavior of two coal fly ashes at well-controlled suspension-fired 2
conditions was investigated through entrained flow reactor (EFR) experiments and chemical 3
equilibrium calculations. The impact of local reaction conditions, i.e., the type of K-salts (K2CO3, KCl 4
or K2SO4), K-concentration in flue gas (molar K/(Al+Si) ratio in reactants), reaction temperature, and 5
coal ash type on the reaction was studied. The results show that the K-capture level of coal fly ash at a 6
K-concentration of 500 ppmv (K/(Si+Al) = 0.481) was considerably lower than the equilibrium data as 7
well as the measured K-capture level of kaolin. However, at 50 ppmv K (with a molar K/(Si+Al) ration 8
of 0.048), no obvious difference between kaolin and coal fly ash was observed in this work. 9
Comparison of results for different K-species showed that coal fly ash captured KOH and K2CO3 more 10
effectively than KCl and K2SO4. Additionally, a coal fly ash with higher content of Si and a lower 11
melting point captured KCl more effectively than the reference coal fly ash. 12
13
Keywords: Coal fly ash, potassium capture, biomass combustion, additive, K2CO3, KCl 14
1 Introduction 15
Biomass suspension-combustion has a higher electrical efficiency and higher load-flexibility 16
compared to traditional grate-fired boilers, but the ash-related problems, including deposition, 17
corrosion and SCR catalyst deactivation, may be more severe [1] than that in grate-fired boilers [2-10], 18
due to a higher concentration of fly ash in the flue gas [6]. Potassium originating from biomass is the 19
primary cause for the ash-related problems. Potassium may be present as KOH, KCl, K2SO4 in the flue 20
3
gas or other forms depending on the fuel composition, ash transformation chemistry, combustion 21
conditions, etc. [3]. In the combustion of woody biomass which contains relatively lower chlorine and 22
sulfur, potassium exists in the flue gas in the boiler chamber mainly as gaseous KOH [11, 12]. When 23
firing straw or other chlorine-rich biomass, chlorine facilitates the release of potassium, and KCl 24
becomes the main K-species in the high temperature flue gas [8, 13]. Apart from accelerating deposit 25
formation and SCR catalyst deactivation, severe corrosion is also attributed to KCl [14-18]. When 26
firing bio-fuels containing sulfur, another K-compound, K2SO4, can be formed [9]. The binary system 27
of KCl and K2SO4 may melt at temperatures as low as 690 °C [19], forming a sticky surface on super-28
heaters and boiler surfaces, which results in accelerated fouling and slagging. 29
Various technologies have been developed to overcome these ash-related problems in biomass-30
fired boilers, including the use of additives [20-28], co-firing [29], leaching [30-33], and application of 31
anti-corrosion coating or materials [34, 35]. Kaolin and coal fly ash are effective additives which can 32
chemically capture K-species forming K-aluminosilicates with higher melting points. 33
Coal fly ash is the only additive that has been commercially utilized in full-scale biomass 34
suspension-fired boilers for K-capture [12, 36]. In a full-scale boiler measuring campaign conducted by 35
Wu and co-workers [36], the influence of the addition of coal fly ash on the transformation of 36
potassium, the deposition behavior, the deposit composition and the formation of sub-micrometer 37
aerosols was systematically investigated [12, 36]. The formation of aerosols was significantly 38
suppressed, and the composition of the aerosols changed from K-S-Cl rich to Ca-P-Si rich [12] with the 39
addition of coal fly ash. The large outer deposit changed from K-Ca-Si rich to Si-Al rich, resulting in 40
an easier and more frequent removal of the deposits [36]. However, due to the complexity of full-scale 41
boiler combustion and the inevitable variation of conditions (bulk chemistry of fuel, load of boiler, etc.), 42
4
it is almost impossible to conduct well-controlled quantitative studies on the K-capture reaction of coal 43
fly ash in full-scale boilers. 44
Some lab-scale experiments have been carried out to understand the K-capture reaction 45
systematically [37, 38]. Zheng et al. [37] studied the KCl capture behavior of coal fly ash pellets in a 46
lab-scale fixed bed reactor, where two types of coal ash were utilized: bituminous coal ash and lignite 47
coal ash. The influences of parent coal type, the reaction temperature, and the K-concentration on the 48
reaction were investigated. The results were also compared with that of kaolin [39], showing that 49
bituminous coal ash with a high content of Al and Si behaved similarly to kaolin and captured KCl 50
effectively. However, the lignite coal ash pellets, which were rich in Ca and Mg, only captured 51
negligible amounts of potassium [37]. 52
In another fixed bed reactor study, Liu et al.[38] investigated the KCl capture reaction by 53
bituminous coal fly ash (70-100 μm) which were paved in a stainless steel wires holder [38]. The 54
impact of reaction temperature, KCl-concentration and the reaction atmosphere was investigated. The 55
results indicated that 900 °C was the optimal K-capture temperature for the investigated coal fly ash. In 56
addition, a reducing atmosphere and the presence of water vapor promoted the K-capture capability of 57
the coal fly ash [38]. 58
Through these fixed bed studies, important data on K-capture by coal fly ash were obtained. 59
However, the reaction conditions in the fixed bed reactors are obviously different from those in full-60
scale suspension-fired boilers [37]. In the fixed bed reactors, coal ash was usually in the form of pellets, 61
flakes, piles or paved in holders [37, 40, 41], causing the reaction with gaseous K-species to be limited 62
by internal diffusion. In suspension-fired boilers, coal ash particles are well-dispersed in the flue gas, 63
having a size smaller than 100 μm, and the controlling mechanism can be quite different. The K-64
5
capture reaction under suspension-fired conditions can be limited by thermal equilibrium, mass transfer, 65
or chemical kinetics. Additionally, local reaction temperature, gas atmosphere, additive particle size, 66
additive composition and reaction time also impact upon the K-capture reaction by coal fly ash [37, 42, 67
43]. However, knowledge on the K-capture reaction of coal ash is limited, and quantitative 68
experimental results on K-capture by coal fly ash at suspension-fired conditions are still not available. 69
Understanding the reaction as well as its relation to local parameters is desirable to achieve an optimal 70
performance of added coal fly ash and model development. 71
The objective of this work is to investigate quantitatively the reaction between coal fly ash and K-72
species at suspension-fired conditions. The impacts of coal ash type, ash particle size, K-species type, 73
K-concentration, and reaction temperature on the K-capture reaction were investigated. This paper is 74
the second one of a series of two papers studying the potassium capture reaction with coal fly ash. The 75
first paper focused on the KOH capture reaction by coal fly ash [44], and the present paper addresses 76
the reaction of coal fly ash with KCl, K2CO3 and K2SO4. 77
2 Experimental 78
2.1 Materials 79
Two types of coal fly ashes were utilized in this study. One was from unit 2 of Asnæsværket Power 80
Plant Denmark, and it was named as ASV2CFA. The other ash sample was from Amager Power Plant 81
and it was named as AMVCFA. Both coal fly ashes were sieved to 0-32 μm, and the sieved samples 82
were named as ASV2CFA0-32 and AMVCFA0-32, respectively. The characteristics of the ash samples 83
are listed in Table 1. Both coal fly ashes have a high content of Al and Si. The molar ratio 84
6
(K+Na)/(Al+Si) of ASV2CFA0-32 and AMVCFA0-32 was 0.02, and 0.07, respectively. Both values 85
are relatively low, indicating that there was a large fraction of Al and Si available for the K-capture 86
reaction. 87
88
Table 1. Characteristics of the coal fly ashes. 89
Name ASV2CFA0-32 AMVCFA0-32
particle size (µm) 0-32 0-32
D50 (µm) 10.20 8.42
O (wt. % dry base) 46.60 49.92
S (wt. % dry base) 0.26 0.23
P (wt. % dry base) 0.64 0.30
Si (wt. % dry base) 22.00 25.00
Al (wt. % dry base) 14.00 11.00
Fe (wt. % dry base) 2.90 4.30
Ca (wt. % dry base) 4.50 4.10
Mg (wt. % dry base) 0.97 1.40
Na (wt. % dry base) 0.27 0.92
K (wt. % dry base) 0.87 2.10
Ti (wt. % dry base) 0.88 0.53
BET surface area (m2/g) 8.04 3.18
deformation temperature (°C) 1280 1200
hemisphere temperature (°C) 1390 1290
flow temperature (°C) 1440 1380
90
One difference between the two coal fly ashes was the alkali metal content. The concentration of 91
(K+Na) in AMVCFA0-32 was about 3.0 wt. %, while it was as low as 1.1 % in ASV2CFA0-32. Alkali 92
elements generally stay in the form of alkali-aluminosilicates in coal ash. A higher content of alkali 93
elements in coal ash may thus reduce the availability of Al and Si for alkali-capture. Another difference 94
7
was that, the Si/Al molar ratio of ASV2CFA0-32 was around 1.5, while the ratio for AMVCFA0-32 95
was 2.2. Usually, Si is present in the form of mullite, quartz or other amorphous species in coal ash. A 96
relatively higher Si or lower Al content usually implies a lower content of mullite, which is considered 97
as a crucial mineral phase in coal ash for K-capture reaction forming K-aluminosilicate [36, 37, 42]. 98
Among the alkaline earth metal elements, the content of Ca in AMVCFA0-32 was slightly lower 99
than that of ASV2CFA0-32, while Mg was slightly higher. Ca is primarily present in coal ash as lime, 100
anhydrite or calcite [45, 46], and it can also exist together with Mg as CaMg-silicate [47, 48]. 101
Therefore, Ca and Mg may also affect the availability of Al and Si, but to a lower extent. In summary, 102
the relatively lower content of Al and higher content of K and Na would be expected to weaken the K-103
capture ability of AMVCFA0-32. 104
In addition to Al and Si, S may also constitute a protective element in coal fly ash, since it can react 105
with KCl or KOH forming less corrosive potassium sulfate [36]. However, the concentration of S in the 106
two selected ashes was very low, around 0.25 %, and may not play a key role in the K-capture reaction 107
in this study. 108
XRD results show that quartz (SiO2) and mullite (3Al2O3·2SiO2) exist in both two coal fly ashes as 109
the main mineral phases. However, no crystalline species containing alkali or alkaline earth metal 110
elements were detected, implying either that the small amount of Na, K, Ca and Mg detected by ICP-111
OES stay in the form of amorphous species, or that the concentrations were too low to be detected. 112
Additionally, the melting points (deformation temperature, hemisphere temperature and flow 113
temperature) of the two coal fly ashes were also analyzed according to ISO540:2008 (Hard coal and 114
coke - Determination of ash fusibility) in an oxidizing atmosphere. The results are listed in Table 1, and 115
it revealed that and the melting points of AMVCFA0-32 are lower than that of ASV2CFA0-32. 116
8
2.2 Experimental methods 117
The DTU Entrained Flow Reactor (EFR) was employed in the experimental work. Detailed 118
information about the reactor is available elsewhere [27, 39]. The experimental conditions are 119
summarized in Table 2. In series A of Table 2, to study the influence of KCl concentration, the 120
concentration of coal fly ash in flue gas was kept constant, while the KCl concentration in flue gas was 121
varied from 50 ppmv to 750 ppmv. In series (B) and (C), the KCl-concentration was kept constant, 122
while the reaction temperature was changed from 800 to 1450 °C, to investigate the influence of 123
reaction temperature. ASV2CFA0-32 and AMVCFA0-32 were utilized in series B and C to compare 124
the KCl capture behavior of the two ashes. The K2CO3 and K2SO4 capturing behavior by coal fly ash at 125
different temperatures was investigated in series (D) and (E). 126
127
9
Table 2. Experimental conditions of K-capture experiments using coal fly ashes in the EFR. 128
Experimental series K-species Additives Temp./°C Gas residence
time/s
K in gas
/ppmv K/(Al+Si)
(A)
KCl-capture by ASV2CFA0-32
(impact of K-concentration)
KCl ASV2CFA0-32 1300 1.2
50* 0.048
250 0.240
500* 0.481
750 0.961
(B)
KCl-capture by ASV2CFA0-32
(impact of temperature)
KCl ASV2CFA0-32
800
1.2 50, 500 0.048, 0.481
900
1100
1300
1450
(C)
KCl-capture by AMVCFA0-32
(impact of temperature)
KCl AMVCFA0-32
800
1.2 500 0.481
900
1100
1300
1450
(D)
K2CO3-capture by ASV2CFA0-32
(impact of temperature)
K2CO3 ASV2CFA0-32
800
1.2 500 0.481 900
1300
(E)
K2SO4-capture by ASV2CFA0-32
(impact of temperature)
K2SO4 ASV2CFA0-32
800
1.2 500 0.481 900
1300*
*Experiments were repeated. 129
10
Table 3. Equilibrium calculation results of KCl capture by ASV2CFA0-32. 130
Input conditions Temp. /°C K-species appearing
Al-
conversion
/%
Si-
conversion
/%
K-
conversion
(XK) /%
K-capture (CK) /(g
K/g additive)
50 ppmv KCl,
K/(Al+Si) =0.048
800 100 % KAlSi3O8 12 24 100 0.023
900 100 % KAlSi3O8 12 24 100 0.023
1100 99 % KAlSi3O8 + 1 % KCl 12 24 99 0.023
1300 97 % KAlSi3O8 + 3 % KCl 12 23 97 0.023
1450 92 % KAlSi3O8 + 7 % KCl + 1 % KOH 11 22 92 0.021
250 ppmv KCl,
K/(Al+Si) =0.240
800 95 % KAlSi2O6 + 5 % KCl 57 77 95 0.111
900 94 % KAlSi2O6 + 6 % KCl 57 76 94 0.110
1100 89 % KAlSi2O6 + 11 % KCl 54 72 89 0.104
1300 84 % KAlSi2O6 + 16 % KCl 50 68 84 0.098
1450 81 % KAlSi2O6 + 18 % KCl + 1 % KOH 49 66 81 0.095
500 ppmv KCl,
K/(Al+Si) =0.481
800 6 % KAlSiO4 + 51 % KAlSi2O6 + 40 % KCl 69 88 57 0.134
900 55 % KAlSi2O6 + 45 % KCl 66 88 55 0.128
1100 47 % KAlSi2O6 + 53 % KCl 57 76 47 0.110
1300 46 % KAlSi2O6 + 53 % KCl + 1 % KOH 56 75 46 0.109
1450 45 % KAlSi2O6 + 54 % KCl + 1 % KOH 54 72 45 0.104
750 ppmv KCl,
K/(Al+Si) =0.721
800 33 % KAlSiO4 + 19 % KAlSi2O6 + 46 % KCl 95 72 52 0.184
900 3 % KAlSiO4 + 35 % KAlSi2O6 + 63 % KCl 68 73 37 0.131
1100 33 % KAlSi2O6 + 67 % KCl 59 65 33 0.114
1300 31 % KAlSi2O6 + 68 % KCl + 1 % KOH 56 62 31 0.109
1450 31 % KAlSi2O6 + 67 % KCl + 2 % KOH 56 62 31 0.109
1000 ppmv KCl,
K/(Al+Si) =0.961
800 29 % KAlSiO4 + 19 % KAlSi2O6 + 57 % KCl 100 87 41 0.193
900 16 % KAlSiO4 + 19 % KAlSi2O6 + 65 % KCl 84 87 35 0.162
1100 25 % KAlSi2O6 + 75 % KCl 60 81 25 0.116
1300 25 % KAlSi2O6 + 75 % KCl + 1 % KOH 56 75 23 0.109
1450 23 % KAlSi2O6 + 75 % KCl + 2 % KOH 56 75 23 0.109
131
11
132
133
Table 4. Summary of the equilibrium calculation results of K2CO3 capture by ASV2CFA0-32. 134
Input conditions Temp. /°C K-species appearing
Al-
conversion
/%
Si-
conversion
/%
K-
conversion
(XK) /%
K-capture (CK) /(g
K/g additive)
250 ppmv K2CO3,
K/(Al+Si) = 0.481
800 73 % KAlSiO4 + 10 % KAlSi2O6 100 76 83 0.194
900 71 % KAlSiO4 + 12 % KAlSi2O6 + 2 % KOH 100 77 83 0.194
1100 55 % KAlSiO4 + 28 % KAlSi2O6 + 15 % KOH 100 90 83 0.194
1300 55 % KAlSiO4 + 28 % KAlSi2O6 + 17 % KOH 100 90 83 0.194
1450 57 % KAlSi2O6 + 42 % KOH 69 92 57 0.133
135
Table 5. Summary of the equilibrium calculation results of K2SO4 capture by ASV2CFA0-32. 136
Input conditions Temp. /°C K-species appearing
Al-
conversion
/%
Si-
conversion
/%
K-
conversion
(XK) /%
K-capture (CK) /(g
K/g additive)
250 ppmv K2SO4,
K/(Al+Si) = 0.481
800 60 % KAlSi2O6 + 40 % K2SO4 73 98 60 0.141
900 59 % KAlSi2O6 + 28 % K2SO4 72 96 59 0.139
1100 54 % KAlSiO4 + 28 % KAlSi2O6 + 4 % KOH 99 89 82 0.191
1300 54 % KAlSiO4 + 28 % KAlSi2O6 + 16 % KOH 100 90 83 0.193
1450 57 % KAlSi2O6 + 42 % KOH 69 92 57 0.133
137
12
The solid products collected from the EFR experiments were analyzed with ICP-OES 138
(Inductively Coupled Plasma Atomic Emission Spectroscopy) to obtain the elemental 139
composition. For the ICP-OES analysis, solid samples were totally digested in acid solution or 140
dissolved in water to determine the total content or the water-soluble content of different 141
elements, including major elements and water soluble elements. The major elements (Al, Ca, Fe, 142
Mg, P, S, K, Si, Na and Ti) were determined according to the Danish Standard of DS/EN 15290 143
(Solid Biofuels - Determination of Major Elements). The concentration of water-soluble 144
elements (K and Cl) was analyzed following the standard of DS/EN ISO 16995 (Solid Biofuels- 145
Determination of water soluble Chloride, Sodium and Potassium). Additionally, XRD (X-ray 146
Diffraction) analysis was employed to get the mineralogical composition of solid products. The 147
XRD spectra were obtained with a Huber diffractometer, and the main crystalline phases were 148
identified with the JADE 6.0 software package (MDI Livermore, CA) and the diffraction 149
database of PDF2-2004. 150
To quantify the K-capture reaction by coal fly ash, two parameters have been defined: K-151
conversion (XK) and K-capture level (CK). XK is the percentage (%) of fed K-species chemically 152
captured by solid additive (coal fly ash) forming water-insoluble K-aluminosilicates. CK is the 153
mass of potassium captured by 1 g of additive (coal fly ash) (g K/g additive). Both two 154
parameters can be calculated based on ICP-OES results, and the detailed calculation method is 155
available in the Appendix I of the supplementary material. 156
157
13
2.3 Equilibrium calculations 158
Equilibrium data were obtained by preforming global chemical equilibrium calculations 159
using FactSage 7.0. The equilibrium calculation results were compared with experimental results 160
to obtain a better understand of the experimental data. But one should note that equilibrium 161
calculation data are obtained assuming a fully mixing of all reactants with an enough long time, 162
which could be not reached in real experiments, and the difference of equilibrium calculation and 163
experimental results can be partly contributed to this. 164
3 Results and discussion 165
3.1 KCl capture by coal fly ash 166
3.1.1 Equilibrium calculations 167
Equilibrium calculation results of KCl capture by ASV2CFA0-32 at 50-1000 ppmv KCl and 168
800-1450 °C were summarized in Table 3. Detailed results of the equilibrium calculations are 169
provided in Appendix II of the supplementary material. The results show that the type of K-170
aluminosilicate formed from the K-capture reaction varied with the changing KCl-concentration 171
and the corresponding molar ratio of K/(Al+Si) in reactants. At 50 ppmv KCl (K/(Al+Si) = 172
0.048), sanidine (KAlSi3O8) was the main K-aluminosilicate. When the KCl concentration 173
increased to 250 ppmv and 500 ppmv (K/(Al+Si) = 0.240 and 0.481), leucite (KAlSi2O6) was 174
predicted to be the dominant K-aluminosilicate at 1100-1450 °C. At 800-900 °C, kaliophilite 175
(KAlSiO4) and leucite (KAlSi2O6) co-existed in the solid products. The equilibrium K-capture 176
level (CK) increased when the KCl concentration changed from 50 ppmv to 500 ppmv. However, 177
14
when the KCl concentration was increased further to 750 and 1000 ppmv, no further increase of 178
equilibrium CK was observed. 179
3.1.2 Impact of KCl concentration 180
To investigate the KCl-capture behavior of coal fly ash at different KCl concentrations, 181
experiments were conducted at 50 ppmv to 750 ppmv KCl, where the molar ratio of K/(Al+Si) in 182
reactants changed from 0.048 to 0.721, correspondingly. The experimental results and 183
equilibrium calculation data are compared in Figure 1. Results for KCl-capture by kaolin from 184
our previous study [39] were also included for comparison. 185
(A) K-capture level (CK) (B) K-conversion (XK)
Figure 1. K-capture level (CK) and K-conversion (XK) of KCl capture by ASV2CFA0-32 at 186
50-750 ppmv KCl (molar ratio of K/(Al+Si) changed from 0.048 to 0.721). Reaction temperature 187
was 1300 °C; the gas residence time was 1.2 s. Experimental data of KCl capture by kaolin from 188
our previous study [39] and equilibrium calculation data of KCl capture by ASV2CFA0-32 were 189
included for comparison. 190
191
0.00
0.05
0.10
0.15
0.20
0 250 500 750 1000
CK
(g K
/ g
add
itiv
e)
K-concentration (ppmv)
Eq. Cal. ASV2CFA0-32ASV2CFA0-32kaolin
0
20
40
60
80
100
0 250 500 750 1000
XK
(%)
K-concentration (ppmv)
EQ kaolin
EQASV2CFA0-32
ASV2CFA0-32
kaolin
15
It is seen from Figure 1, that the measured K-capture level (CK) of ASV2CFA0-32 increased 192
from 0.019 g K/(g additive) to 0.041 g K/(g additive), when the KCl concentration increased 193
from 50 ppmv to 500 ppmv. Measured K-conversion (XK) of ASV2CFA0-32 decreased from 194
80.0 % to 17.5 % correspondingly. However, when the KCl-concentration increased further to 195
750 ppmv and 1000 ppmv, CK did not increase, with XK decreased further to 11.7 %. Comparing 196
to the equilibrium calculation results, the measured CK and XK was considerably lower. This 197
implied that the KCl-ASV2CFA0-32 reaction was far from reaching chemical equilibrium 198
probably due to internal diffusion limitations of KCl. 199
Comparing the CK and XK of ASV2CFA0-32 with kaolin [39] in Figure 1, at 250 ppmv KCl 200
and above, the experimental CK and XK of ASV2CFA0-32 were remarkably lower than that of 201
kaolin [39]. The lower BET surface area (8.04 m2/g) and the relatively bigger particle size (D50 = 202
10.20 μm) of ASV2CFA0-32 compared with kaolin (BET surface area = 12.70 m2/g, D50 = 5.47 203
μm) was one possible reason; another possible reason being the lower reactivity of mullite in 204
coal fly ash towards potassium, compared to kaolinite [37]. At 50 ppmv KCl (K/(Al+Si) in 205
reactants was 0.048), the measured CK and XK of ASV2CFA0-32 were comparable to those of 206
kaolin [39]. The results indicated that at low K-concentrations (50 ppmv) or low K/(Al+Si) 207
molar ratio (0.048), which is representative for the gaseous potassium level in practical wood 208
suspension-fired plants [36, 49-51], the K-capture capacity of kaolin and coal fly ash is similar. 209
This is probably because at lower K/(Al+Si), the mullite in the surface layer of coal fly ash 210
particles is probably sufficient for capturing the low amount of potassium, therefore the reaction 211
is less influenced by the internal diffusion of KCl. 212
213
16
214 Figure 2. XRD spectra of water-washed KCl-reacted ASV2CFA0-32 at 50 ppmv, 250 ppmv 215
and 500 ppmv KCl. The reaction temperature was 1300 °C. The molar ratio of K/(Al+Si) in the 216
reactants changed from 0.048 to 0.481. The gas residence time was 1.2 s. XRD spectrum of coal 217
fly ash without K feeding was also included for comparison. 218
219
The XRD results of water-washed KCl-reacted ASV2CFA0-32 are shown in Figure 2. It 220
shows that the XRD spectrum of 50 ppmv KCl-reacted ash is almost identical as that of coal ash 221
without K feeding, and no crystalline K-aluminosilicate was detected, although sanidine 222
(KAlSi3O8) was predicted by the equilibrium calculations (Table 3) and some water-insoluble 223
potassium was detected by ICP-OES analysis. This is probably because K-aluminosilicate 224
products existed in amorphous phase or its content was too low to be detected. Leucite 225
(KAlSi2O6) was detected both in the 250 ppmv and 500 ppmv KCl-reacted ash samples. This 226
10 20 30 40 50 60 70 80 90 100
0
2500
5000
7500
Inte
nsi
ty (
cou
nts
)
50 ppmv
250 ppmv
500 ppmv
M Q
M
Q
MMM
Q
M
MM M QMM M
ML+
Q
L LQ
LLMM MM Q ML MM M Q
L - leucite KAlSi2O6
Q - quartz SiO2
M - mullite 3Al2O3· 2SiO2
ML+
Q
L LQ
LL
M M MM
LQ M MM M QM
raw ash
2θ (degrees)
M Q
M
Q
M MMQ
MMM M QM M M
17
agrees with the equilibrium prediction shown in Table 3. Additionally, the types of K-227
aluminosilicate detected also agreed with what was observed in KCl-kaolin reaction in our 228
previous study [39]. Notably, in addition to K-aluminosilicate, mullite (3Al2O3·2SiO2) and 229
quartz (SiO2) were also detected in all the KCl-reacted ash samples, indicating that some mullite 230
and quartz originating from the parental coal fly ash remained unreacted. This is presumably the 231
reason why the measured K-capture level (CK) of ASV2CFA0-32 was remarkably lower than the 232
equilibrium prediction. 233
3.1.3 Impact of reaction temperature 234
The K-capture level (CK) and K-conversion (XK) of KCl capture by ASV2CFA0-32 and 235
AMVCFA0-32 at different temperatures are shown in Figure 3 and Figure 4, respectively. For 236
ASV2CFA0-32, experiments were conducted at both 50 ppmv and 500 ppmv KCl. For 237
AMVCFA0-32, experiments were only conducted with a KCl concentration of 500 ppmv. 238
239
18
240
(A) K-capture level (CK) at 50 ppmv KCl (B) K-conversion (XK) at 50 ppmv KCl
(C) K-capture level (CK) at 500 ppmv KCl (D) K-conversion (XK) at 500 ppmv KCl
Figure 3. K-capture level (CK) and K-conversion (XK) of KCl-capture by ASV2CFA0-32 at 241
800-1450 °C. KCl-concentration was 50 ppmv (molar ratio of K/(Al+Si) = 0.048) in (A) and (B), 242
and 500 ppmv (molar ratio of K/(Al+Si) = 0.481) in (C) and (D). The gas residence time was 1.2 243
s. Equilibrium calculation results are included for comparison. 244
245
As shown in Figure 3(A) and (B), at 50 ppmv KCl (K/(Al+Si) = 0.048), the measured CK 246
and XK of ASV2CFA0-32 were close to the equilibrium calculation data and no obvious change 247
0.00
0.05
0.10
0.15
0.20
700 900 1100 1300 1500
CK
(g K
/ g
add
itiv
e)
Temperature (°C)
Equilibrium calculation
EFR experiments
0
20
40
60
80
100
700 900 1100 1300 1500
XK
(%)
Temperature (°C)
Equilibrium calculation
EFR experiments
0.00
0.05
0.10
0.15
0.20
700 900 1100 1300 1500
CK
(g K
/ g
add
itiv
e)
Temperature (°C)
Equilibrium Calculation
EFR experiments
0
20
40
60
80
100
700 900 1100 1300 1500
XK
(%)
Temperature (°C)
Equilibrium Calculation
EFR experiment
19
of CK was observed within the studied temperature range (800-1450 °C). CK was around 0.018 g 248
K/(g additive), with about 80 % of the potassium fed captured by coal fly ash. 249
Figure 3(C) and (D) show that, at 800 °C and 500 ppmv KCl (K/(Al+Si) = 0.481), the 250
experimental CK is fairly low (0.015 g K/(g additive)). However, when the reaction temperature 251
increased to 900 °C, the experimental CK increased significantly to 0.035 g K/(g additive). In the 252
temperature range 900-1100 °C, no significant change of CK and XK was observed. We believe 253
that this is because, at 800 °C, the KCl-coal fly ash reaction was probably kinetically controlled, 254
and it was less kinetically influenced at 900-1100 °C. As the reaction temperature increased 255
further to 1300 and 1450 °C, CK increased gradually to 0.053 g K/(g additive). However, the 256
experimental CK and XK were both obviously lower than the equilibrium predictions. Noticing 257
the vaporization degree of KCl (500 ppmv) at different temperatures from 800 to 1450 °C in the 258
EFR was similar (95.4-99.7%) [39]. The increase of CK at 900-1450 °C, especially at 1300 °C 259
and 1450 °C, is probably due to the melting of coal ash particles (deformation temperature of 260
ASV2CFA0-32 was 1280 °C), which enhanced the KCl diffusion inside the particle (D50 = 10.20 261
μm). A similar phenomenon was observed in the KCl capture experiments using AMVCFA0-32, 262
as discussed below. 263
Another interesting result in Figure 3 is that, at 800 °C, CK at 500 ppmv KCl (0.015 g K/(g 264
additive)) is comparable to that at 50 ppmv KCl (0.014 g K/(g additive)). It shows that, at 800 °C, 265
increasing the KCl-concentration from 50 to 500 ppmv did not elevate the amount of potassium 266
captured by coal fly ash under the studied condition. This is probably because the reaction at 267
800 °C was kinetically controlled and the KCl concentration did not to a large degree influence 268
the reaction. 269
20
The experimental CK and XK for KCl-capture by AMVCFA0-32 are shown in Figure 4 (A) 270
and (B). The trend of CK and XK of AMVCFA0-32 at different temperatures was similar to that 271
of ASV2CFA0-32. At 800 °C, CK was as low as 0.015 g K/(g additive), and it increased to 272
around 0.030 g K/(g additive) at 900 and 1100 °C. When the temperature increased further to 273
1300 and 1450 °C, CK increased considerably to 0.069 g K/(g additive). 274
(A) K-capture level of AMVCFA0-32 (B) K-conversion of AMVCFA0-32
Figure 4. K-capture level (CK) and K-conversion (XK) of KCl capture by AMVCFA0-32 at 275
different temperatures (800-1450 °C). KCl concentration was 500 ppmv with molar ratio of 276
K/(Al+Si) = 0.481 in reactants. The gas residence time was 1.2 s. Equilibrium calculation results 277
and fixed bed reactor data* (bituminous coal ash pellets with diameter of 1.5 mm, 1100 °C, 1000 278
ppmv KCl, residence time was 1 hour) calculated from literature [37] are included for 279
comparison. 280
281
The experimental CK for KCl capture by bituminous coal fly ash pellets (diameter of 1.5 mm 282
in a fixed bed reactor) from literature [37] is included in Figure 4 (A) for comparison. The KCl 283
concentration in the fixed bed reactor was 1000 ppmv, and the residence time was 1 hour, i.e., 284
much longer than that in the EFR (1.2 s) of this study. It is seen that CK in the fixed bed reactor 285
0.00
0.05
0.10
0.15
0.20
700 900 1100 1300 1500
CK
(g K
/ g
add
itiv
e)
Temperature (°C)
Equilibrium calculationEFR experimentsFixed bed*
0
20
40
60
80
100
700 900 1100 1300 1500
XK
(%)
Temperature (°C)
Equilibrium Calculation
EFR experiments
21
from literature was considerably higher than that in the EFR at 800-1100 °C. This is because the 286
longer residence time in the fixed bed reactor favored the reaction and more Al and Si from coal 287
fly ash participated in the KCl capture reaction. However, at 1300 °C, the CK in fixed bed and 288
EFR became comparable despite the difference in residence time and KCl concentration. 289
Possibly, this is because the melting of the ash particles at high temperature (1300 °C and 290
1450 °C) made the reaction in the EFR less diffusion-influenced. 291
3.1.4 Impact of coal fly ash type 292
The experimental CK of the two coal ashes (ASV2CFA0-32 and AMVCFA0-32) as well as 293
that of kaolin from our previous study [39] are compared in Figure 5. The equilibrium 294
calculation data of KCl capture by the two coal fly ashes were also included. Below 1100 °C, CK 295
and XK of ASV2CFA0-32 and AMVCFA0-32 were similar, whereas at 1300 °C and 1450 °C, 296
AMVCFA0-32 captured KCl more effectively than ASV2CFA0-32, despite its higher content of 297
K and Na. One possible explanation is that the melting point of AMVCFA0-32 is lower than that 298
of ASV2CFA0-32 as shown in Table 1. The melting of the ash particles presumably facilitates 299
internal diffusion of KCl. Similar phenomena, that the K-capture amount by coal fly ash 300
increased at 1200 °C and above, was observed by Zheng in a fixed bed study of KCl capture by 301
coal fly ash pellets [37]. Another possible reason is that the Si concentration in AMVCFA0-32 is 302
higher than that in ASV2CFA0-32. The higher Si content facilitated the formation of leucite 303
(KAlSi2O6) (K:Al:Si = 1:1:2) in the KCl-coal fly ash reaction. A higher CK of AMVCFA0-32 304
was observed both in the equilibrium calculations and the EFR experiments at 1300 °C and 305
1450 °C. 306
307
22
308
Figure 5. Comparison of CK of KCl capture by ASV2CFA0-32, AMVCFA0-32 and kaolin 309
[39] at different temperature. KCl-concentration was 500 ppmv with molar ratio of K/(Al+Si) = 310
0.481, gas residence time was 1.2 s. Equilibrium calculation data of KCl capture by ASV2CA0-311
32 and AMVCFA0-32 were included for comparison. 312
313
3.2 K2CO3 capture by coal fly ash 314
3.2.1 Equilibrium calculation 315
Equilibrium calculations of K2CO3 capture by ASV2CFA0-32 were conducted with a K2CO3 316
concentration of 250 ppmv (K-concentration in flue gas was 500 ppmv), and reaction 317
temperatures changing from 500 °C to 1800 °C. The equilibrium calculation results are 318
summarized in Table 4, and detailed data can be found in Appendix II of the supplementary 319
material. 320
The equilibrium calculation results generally agreed with the prediction for KOH-capture by 321
ASV2CFA0-32 in our previous study [44]. At 250 ppmv K2CO3 (K/(Al+Si) = 0.481) and 800-322
0.00
0.05
0.10
0.15
700 900 1100 1300 1500
K-c
aptu
re le
vel
CK
(g K
/ g
add
itiv
e)
Temperature (°C)
Kaolin
EQ AMV
EQ ASV2
Exp AMV
Exp ASV2
23
1300 °C, kaliophilite (KAlSiO4) was predicted to be the dominant K-aluminosilicate in products, 323
together with some (leucite) KAlSi2O6. At 1450 °C, leucite (KAlSi2O6) was present as the 324
dominant K-aluminosilicate product. The equilibrium CK and XK was constant at 800-1300 °C, 325
and a decreased CK was predicted at 1450 °C. 326
3.2.2 Impact of reaction temperature 327
The measured CK and XK of K2CO3 capture by ASV2CFA0-32 are compared to the 328
equilibrium calculations in Figure 6, under the conditions of 800-1300 °C, 250 ppmv K2CO3 and 329
a gas residence time of 1.2 s. The experimental CK and XK of K2CO3 capture by kaolin (D50 = 330
5.47 μm) from our previous study [39] were included for comparison. We believe that at the 331
applied temperatures (800°C and above) K2CO3 decomposes to KOH that reacts with the coal fly 332
ash. 333
334
24
(A) K-capture level CK (B) K-conversion XK
Figure 6. K-capture level (CK) and K-conversion (XK) of K2CO3 capture by ASV2CFA0-32 335
at 800-1300 °C. K2CO3 concentration was 250 ppmv (molar ratio of K/(Al+Si) = 0.481). The gas 336
residence time was 1.2 s. Equilibrium calculation results of K2CO3 capture by ASV2CFA0-32, 337
and experimental results of K2CO3-capture by kaolin from our previous study [39] are included 338
for comparison. 339
340
According to our previous work, the vaporization degree of K2CO3 at 800-1450 °C in the 341
EFR was similar (97.3-99.7 %). However, experimental CK (0.025 g K/(g additive)) at 800 °C 342
was much lower than that at 900 °C (0.056 g K/(g additive)). The significant difference is 343
probably because the reaction was kinetically controlled at 800 °C. When the reaction 344
temperature increased to 1300 °C, CK increased slightly to 0.070 g K/(g additive). The 345
experimental XK had the same trend as that of CK, and it was below 30 % throughout the whole 346
temperature range studied. 347
The results also show that the experimental CK and XK of ASV2CFA0-32 were significantly 348
lower than that of kaolin [39] and the data predicted by equilibrium calculations. The lower BET 349
0.00
0.05
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0.25
0.30
700 900 1100 1300 1500
CK
(g K
/ g
add
itiv
e)
Temperature (°C)
Eq. Cal. ASV2CFA0-32ASV2CFA0-32kaolin
0
20
40
60
80
100
700 900 1100 1300 1500
XK
(%)
Temperature (°C)
Eq. Cal. ASV2CFA0-32
ASV2CFA0-32
Kaolin
25
surface area of coal fly ash (8.04 m2/g) than that of kaolin (12.70 m
2/g), and the bigger particle 350
size of ASV2CFA0-32 (D50 = 12.70 μm) than that of kaolin (D50 = 5.47 μm) may cause some of 351
the difference. Another possible reason is that the main mineral phase in ASV2CFA0-32, mullite, 352
was less active towards K2CO3. Additionally, the relatively lower Al content of ASV2CFA0-32 353
may have contributed to the lower CK as well. 354
355
Figure 7. XRD spectra of water-washed K2CO3-reacted ASV2CFA0-32. K2CO3 356
concentration in flue gas was 250 ppmv; molar ratio of K/(Al+Si) in reactants was 0.481. Gas 357
residence time was 1.2 s. 358
The XRD spectra of water-washed K2CO3-reacted ASV2CFA0-32 at different temperatures 359
were compared in Figure 7. In the 1300 °C sample, kaliophilite (KAlSiO4) was detected together 360
with mullite and quartz. However, in the 800 °C and 900 °C samples, no crystalline K-361
aluminosilicate was detected although the ICP-OES analysis showed that the experimental CK at 362
10 20 30 40 50 60 70 80 90 100
Two-Theta (deg)
0
1000
2000
3000
4000
5000
6000
7000
Inte
nsity(C
ounts
)
2θ (degrees)
Inte
nsi
ty (
cou
nts
)
800 °C
900 °C
1300 °C
M QM
Q
MMMQ
M
MMM QM
M M
M Q
Q
M KlKl
M M MM M
MMQ
Kl - kaliophilite KAlSiO4
Q - quartz SiO2
M - mullite 3Al2O3· 2SiO2
M QM
Q
MMMQ
M
MMM M
M M
Q
M
26
900 °C was similar as that of 1300 °C. This is probably because, at 900 °C, only amorphous K-363
aluminosilicate was formed, and it cannot be detected by XRD. 364
3.3 K2SO4 capture by coal fly ash 365
3.3.1 Equilibrium calculation 366
Equilibrium calculations of K2SO4 capture by ASV2CFA0-32 were conducted at 250 ppmv 367
K2SO4 and a temperature range from 500 °C to 1800 °C. The equilibrium calculation results are 368
summarized in Table 5. Detailed results are provided in Appendix II of the supplementary 369
material. 370
The equilibrium calculations show that at 800 °C, 900 °C and 1450 °C, leucite (KAlSi2O6) 371
was predicted to be the dominant K-aluminosilicate product. At 1100 °C and 1300 °C, 372
kaliophilite (KAlSiO4) was predicted to be present as the main K-aluminosilicate in the product. 373
The calculated CK firstly increased and then decreased with the increasing temperature in the 374
studied temperature range. 375
3.3.2 Impact of temperature 376
The experimental CK and XK of K2SO4 capture by ASV2CFA0-32 are compared with 377
equilibrium calculations as well as the experimental CK and XK of K2SO4 capture by kaolin [39] 378
in Figure 8. At 800 °C, CK of ASV2CFA0-32 was 0.013 g K/(g additive), with only 5.7 % K2SO4 379
converted into K-aluminosilicate. The low conversion was partly because of an incomplete 380
vaporization of K2SO4, and partly because of that the reaction was slow at 800 °C. At 900 °C and 381
1300 °C, CK increased to 0.025 g K/(g additive) and 0.037 g K/(g additive) respectively. 382
However, similar to what was observed for KCl and K2CO3 capture by ASV2CFA0-32, the 383
27
measured CK and XK of K2SO4 were remarkably lower than the equilibrium data. This is because 384
the fly ash only partly reacted, with some mullite remaining unreacted in products. This was 385
supported by the XRD results discussed below. 386
(A) K-capture level (CK) (B) K-conversion (XK)
Figure 8. K-capture level (CK) and K-conversion (XK) of K2SO4 capture by ASV2CFA0-32 387
at temperatures 800-1300 °C. K2SO4 concentration in flue gas was 250 ppmv (K-concentration 388
in flue gas is 500 ppmv), and molar ratio of K/(Al+Si) in reactants was 0.481. The gas residence 389
time was 1.2 s. Equilibrium calculation results of K2SO4 capture by ASV2CFA0-32, and 390
experimental CK of K2SO4 capture by kaolin [39] are included for comparison. 391
392
0.00
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0.25
0.30
700 900 1100 1300 1500
CK
(g K
/ g
add
itiv
e)
Temperature (°C)
Eq. Cal. ASV2CFA0-32KaolinASV2CFA0-32
0
20
40
60
80
100
700 900 1100 1300 1500X
K(%
)Temperature (°C)
Eq. Cal. ASV2CFA0-32KaolinASV2CFA0-32
28
393
Figure 9. XRD spectra of water-washed K2SO4-reacted ASV2CFA0-32 at different 394
temperatures (800, 900 and 1300 °C). K2SO4 concentration was 250 ppmv; molar K/(Al+Si) 395
ratio in reactants was 0.481. The gas residence time was 1.2 s. 396
The XRD spectra of water-washed K2SO4-reacted ASV2 coal fly ash are compared in Figure 397
9. At 800 °C and 900 °C, only mullite and quartz were detected in the products, with no 398
indication of crystalline K-aluminosilicates. In the 1300 °C sample, leucite (KAlSi2O6) was the 399
only K-aluminosilicate detected, although kaliophilite (KAlSiO4) and leucite (KAlSi2O6) were 400
predicted to co-exist by the equilibrium calculation. Similar results were observed in the K2SO4-401
kaolin reaction using in our previous study [39]. 402
10 20 30 40 50 60 70 80 90 100
Two-Theta (deg)
0
1000
2000
3000
4000
5000
6000
7000
Inte
nsity(C
ounts
)
2θ (degrees)
Inte
nsi
ty (
cou
nts
)
800 °C
900 °C
1300 °C
M QM
Q
MMMQ
MMM M QM
M M
ML+
Q
L LQ
L LM M M
MM Q MLMM M Q
L - leucite KAlSi2O6
Q - quartz SiO2
M - mullite 3Al2O3· 2SiO2
M Q
M
Q
MMMQ
MMM M QM
M M
29
3.4 Comparison of different K-species 403
Reaction between ASV2CFA0-32 and different K-species (KOH, K2CO3, KCl and K2SO4) is 404
compared in Figure 10. The results of KOH-ASV2CFA0-32 reaction are from our previous study 405
[44]. It shows that the K-capture level (CK) for KOH and K2CO3 by ASV2CFA0-32 were very 406
similar (0.05-0.07 g K/(g additive)). We attribute this to a rapid conversion of K2CO3 to KOH in 407
the reactor followed by reaction of KOH with ASV2CFA0-32. A similar behavior was observed 408
in our previous study where KOH and K2CO3 capture by kaolin was investigated [39]. The trend 409
of CK of K2SO4 capture by ASV2CFA0-32 at different temperatures generally agreed with that 410
of KCl (CK = 0.02-0.04 g K/(g additive)). Similar results were also seen in our previous study of 411
KCl and K2SO4 capture by kaolin [39]. ASV2CFA0-32 captured KOH and K2CO3 more 412
effectively than KCl and K2SO4 in the studied temperature range and K-concentration. 413
414
Figure 10. Comparison of CK of K-capture by ASV2CFA0-32 using different K-species 415
(KOH, K2CO3, KCl and K2SO4). The K-concentration was 500 ppmv; molar K/(Al+Si) ratio in 416
reactants was 0.481. The gas residence time was 1.2 s. *Data of KOH capture by ASV2CFA0-32 417
was from our previous study [44]. 418
419
0.00
0.05
0.10
0.15
700 900 1100 1300 1500
CK
(g K
/ g
add
itiv
e)
Temperature (°C)
KOH*K₂CO₃
KCl
K₂SO₄
30
The results imply that in the case of capturing KCl or K2SO4, more additives may be needed 420
to achieve a satisfactory K-capture. The reason for this is that at high temperatures the main 421
product of the reaction with KCl or K2SO4, is leucite (KAlSi2O6) while the main product of 422
reactions with KOH and K2CO3 is kaliophilite (KAlSiO4). In addition, coal fly ash with a 423
relatively higher content of Si seems more suitable than coal fly ash with a similar Al and Si 424
contents for K-capture when burning Cl-rich biomass fuels. 425
4 Conclusions 426
The K-capture behavior of two coal fly ashes were studied by conducting experiments in an 427
entrained flow reactor and doing chemical equilibrium calculations. The influence of the type of 428
K-species, the K-concentration in flue gas (molar ratio of K/(Al+Si) in reactants), reaction 429
temperature, as well as the type of coal fly ashes on the K-capture reaction was systematically 430
investigated. 431
For KCl at 1300 °C, the K-capture level (CK) of coal fly ashes increased from 0.02 g K/(g 432
additive) to 0.04 g K/(g additive) when the KCl concentration increased from 50 ppmv to 500 433
ppmv (molar ratio of K/(Al+Si) in reactants increased from 0.048 to 0.481). However, CK did not 434
increase when the KCl concentration increased further to 750 ppmv (molar ratio of K/(Al+Si) = 435
0.721). 436
At 800 °C, the K-capture reaction was kinetically limited and a relatively low K-capture 437
level (CK) was observed for all studied K-species (KOH, KCl, K2CO3 and K2SO4). At 900 °C 438
and up to 1450 °C, CK generally increased with increasing reaction temperature for all the 439
applied K-species. Possibly the melting of coal fly ash at high temperature (1300 and 1450 °C) 440
31
enhanced the internal diffusion of K-species, and resulted a higher CK values. KOH and K2CO3 441
had similar CK levels of 0.05-0.07 g K/(g additive), and KCl and K2SO4 obtained CK levels of 442
0.02-0.04 g K/(g additive) in the temperature range from 900 to 1450°C (with a K-concentration 443
of 500 ppmv, molar K/(Al+Si) ratio in reactants of 0.481, and a residence time of 1.2 s). At high 444
temperature (1300 °C) crystalline kaliophilite (KAlSiO4) was detected in K2CO3-reacted coal fly 445
ash, but leucite (KAlSi2O6) were detected in KCl and K2SO4-reacted coal fly ashes. In addition, 446
mullite was detected in reacted coal fly ashes by XRD, showing that coal fly ash remained only 447
partially reacted in the product samples. 448
The CK and XK levels of the two coal fly ashes were compared with that of kaolin from our 449
previous studies [27, 39]. CK of the two coal fly ashes was obviously lower than that of kaolin at 450
500 ppmv K (K/(Al+Si) = 0.481). However, at 50 ppmv K (K/(Al+Si) = 0.048), which is 451
comparable to the conditions in full-scale wood suspension-fired boilers, CK of kaolin and coal 452
fly ash was similar. The AMVCFA0-32 coal ash with a lower melting point and high Si content 453
captured more KCl than ASV2CFA0-32, probably because the internal diffusion of KCl inside 454
the AMV coal ash particles was enhanced by the melting of the coal ash particles, and the high 455
Si content facilitated the formation of leucite (KAlSi2O6). 456
Based on the results obtained from this study, some guidelines on using additives in full-457
scale PF-boilers are summarized below. 458
The composition of coal fly ash can affect the K-capture behavior. Bituminous coal 459
ash with high Al and Si contents are preferred as K-capture additive. 460
Coal fly ash captures potassium from woody biomass more effectively than from 461
straw (Cl-rich). Dosage of coal ash should be increased when firing herbaceous 462
biomass containing Cl or S, like, straw. 463
32
High-temperature can enhance the K-capture reaction by coal fly ash. Premixing fuel 464
with coal fly ash and feed the mixture into boilers is preferred, since fully mixing 465
and high temperatures can both be obtained. 466
5 Acknowledgements 467
This work is part of the project ‘Flexible use of Biomass on PF fired power plants’ funded 468
by Energinet.dk through the ForskEL programme, Ørsted Bioenergy & Thermal Power A/S and 469
DTU. 470
Supplementary material. Appendix I: detailed information about quantification method of 471
the K-capture reaction. Appendix II: detailed information about the equilibrium calculation. In 472
Appendix II: part A is the detailed results of the equilibrium calculations of KCl capture by 473
ASV2CFA0-32; Part B is the detailed results of the equilibrium calculations of K2CO3 capture 474
by ASV2CFA0-32; Part C is the detailed results of the equilibrium calculations of K2SO4 capture 475
by ASV2CFA0-32; Part D is the detailed results of the equilibrium calculations of KCl capture 476
by AMVCFA0-32. 477
33
6 References 478
[1] Wu H, Glarborg P, Frandsen F J, Dam-Johansen K, Jensen P A. Dust-Firing of Straw and 479
Additives: Ash Chemistry and Deposition Behavior. Energy Fuels 2011; 25: 2862-2873. 480
[2] Vassilev S V, Baxter D, Andersen L K, Vassileva C G. An overview of the composition and 481
application of biomass ash. Fuel 2013; 105: 19-39. 482
[3] Johansen J M, Aho M, Paakkinen K, Taipale R, Egsgaard H, Jakobsen J G, Frandsen F J, 483
Glarborg P. Release of K, Cl, and S during combustion and co-combustion with wood of high-484
chlorine biomass in bench and pilot scale fuel beds. Proc. Combust. Inst. 2013; 34: 2363-2372. 485
[4] Vassilev S V, Baxter D, Andersen L K, Vassileva C G, Morgan T J. An overview of the 486
organic and inorganic phase composition of biomass. Fuel 2012; 94: 1-33. 487
[5] Vassilev S V, Baxter D, Andersen L K, Vassileva C G. An overview of the chemical 488
composition of biomass. Fuel 2010; 89: 913-933. 489
[6] Frandsen F J, Ash Formation, Deposition and Corrosion When Utilizing Straw for Heat and 490
Power Production, in: Department of Chemical and Biochemical Engineering, Technical 491
University of Denmark, Denmark, 2011. 492
[7] Laxminarayan Y, Jensen P A, Wu H, Frandsen F J, Sander B, Glarborg P. Deposit Shedding 493
in Biomass-Fired Boilers: Shear Adhesion Strength Measurements. Energy Fuels 2017; 31: 494
8733-8741. 495
[8] Damoe A J, Jensen P A, Frandsen F J, Wu H, Glarborg P. Fly Ash Formation during 496
Suspension Firing of Biomass: Effects of Residence Time and Fuel Type. Energy Fuels 2017; 497
31: 555-570. 498
34
[9] Li L, Yu C, Huang F, Bai J, Fang M, Luo Z. Study on the Deposits Derived from a Biomass 499
Circulating Fluidized-Bed Boiler. Energy Fuels 2012; 26: 6008-6014. 500
[10] Anicic B, Lin W, Dam-Johansen K, Wu H. Agglomeration mechanism in biomass fluidized 501
bed combustion – Reaction between potassium carbonate and silica sand. Fuel Process. Technol. 502
2018; 173: 182-190. 503
[11] Niu Y, Tan H, Hui S e. Ash-related issues during biomass combustion: Alkali-induced 504
slagging, silicate melt-induced slagging (ash fusion), agglomeration, corrosion, ash utilization, 505
and related countermeasures. Prog. Energy Combust. Sci. 2016; 52: 1-61. 506
[12] Damoe A J, Wu H, Frandsen F J, Glarborg P, Sander B. Impact of Coal Fly Ash Addition 507
on Combustion Aerosols (PM 2.5 ) from Full-Scale Suspension-Firing of Pulverized Wood. 508
Energy Fuels 2014; 28: 3217-3223. 509
[13] Wang G, Shen L, Sheng C. Characterization of Biomass Ashes from Power Plants Firing 510
Agricultural Residues. Energy Fuels 2012; 26: 102-111. 511
[14] Nielsen H P, Frandsen F J, Dam-Johansen K, Baxter L L. The implications of chlorine-512
associated corrosion on the operation of biomass-fired boilers. Prog. Energy Combust. Sci. 513
2000; 26: 283-298. 514
[15] Nielsen H. Deposition of potassium salts on heat transfer surfaces in straw-fired boilers: a 515
pilot-scale study. Fuel 2000; 79: 131-139. 516
[16] Hansen L A, Nielsen H P, Frandsen F J, Dam-Johansen K, Hørlyck S, Karlsson A. Influence 517
of deposit formation on corrosion at a straw-fired boiler. Fuel Process. Technol. 2000; 64: 189-518
209. 519
[17] Zheng Y, Jensen A D, Johnsson J E. Deactivation of V2O5-WO3-TiO2 SCR catalyst at a 520
biomass-fired combined heat and power plant. Appl. Catal., B: Environ. 2005; 60: 253-264. 521
35
[18] Zheng Y, Jensen A D, Johnsson J E, Thøgersen J R. Deactivation of V2O5-WO3-TiO2 SCR 522
catalyst at biomass fired power plants: Elucidation of mechanisms by lab- and pilot-scale 523
experiments. Appl. Catal., B: Environ. 2008; 83: 186-194. 524
[19] Lindberg D, Backman R, Chartrand P. Thermodynamic evaluation and optimization of the 525
(NaCl+Na2SO4+Na2CO3+KCl+K2SO4+K2CO3) system. The Journal of Chemical 526
Thermodynamics 2007; 39: 1001-1021. 527
[20] Aho M, Vainikka P, Taipale R, Yrjas P. Effective new chemicals to prevent corrosion due to 528
chlorine in power plant superheaters. Fuel 2008; 87: 647-654. 529
[21] Davidsson K O, Åmand L E, Steenari B M, Elled A L, Eskilsson D, Leckner B. 530
Countermeasures against alkali-related problems during combustion of biomass in a circulating 531
fluidized bed boiler. Chem. Eng. Sci. 2008; 63: 5314-5329. 532
[22] Wang L, Skjevrak G, Hustad J E, Grønli M, Skreiberg Ø. Effects of additives on barley 533
straw and husk ashes sintering characteristics. Energy Procedia 2012; 20: 30-39. 534
[23] Wang L, Hustad J E, Skreiberg Ø, Skjevrak G, Grønli M. A Critical Review on Additives to 535
Reduce Ash Related Operation Problems in Biomass Combustion Applications. Energy Procedia 536
2012; 20: 20-29. 537
[24] Xu L, Liu J, Kang Y, Miao Y, Ren W, Wang T. Safely Burning High Alkali Coal with 538
Kaolin Additive in a Pulverized Fuel Boiler. Energy Fuels 2014; 28: 5640-5648. 539
[25] Steenari B M, Lindqvist O. High-temperature reactions of straw ash and the anti-sintering 540
additives kaolin and dolomite. Biomass Bioenergy 1998; 14: 67-76. 541
[26] De Fusco L, Boucquey A, Blondeau J, Jeanmart H, Contino F. Fouling propensity of high-542
phosphorus solid fuels: Predictive criteria and ash deposits characterisation of sunflower hulls 543
with P/Ca-additives in a drop tube furnace. Fuel 2016; 170: 16-26. 544
36
[27] Wang G, Jensen P A, Wu H, Frandsen F J, Sander B, Glarborg P. Potassium Capture by 545
Kaolin, Part 1: KOH. Energy Fuels 2018; 32: 1851-1862. 546
[28] Fuller A, Omidiji Y, Viefhaus T, Maier J, Scheffknecht G. The impact of an additive on fly 547
ash formation/transformation from wood dust combustion in a lab-scale pulverized fuel reactor. 548
Renewable Energy 2019; 136: 732-745. 549
[29] Zheng Y, Jensen P A, Jensen A D, Sander B, Junker H. Ash transformation during co-firing 550
coal and straw. Fuel 2007; 86: 1008-1020. 551
[30] Dayton D C, Jenkins B M, Turn S Q, Bakker R R, Williams R B, Belle-Oudry D, Hill L M. 552
Release of Inorganic Constituents from Leached Biomass during Thermal Conversion. Energy 553
Fuels 1999; 13: 860-870. 554
[31] Jenkins B M, Bakker R R, Wei J B. On the properties of washed straw. Biomass Bioenergy 555
1996; 10: 177-200. 556
[32] Turn S, Kinoshita C, Ishimura D, Jenkins B, Zhou J. Leaching of Alkalis in Biomass Using 557
Banagrass as a Prototype Herbaceous Species, National Renewable Energy Laboratory, 558
California, 2003. 559
[33] Davidsson K O, Korsgren J G, Pettersson J B C, Jäglid U. The effects of fuel washing 560
techniques on alkali release from biomass. Fuel 2002; 81: 137-142. 561
[34] Oksa M, Auerkari P, Salonen J, Varis T. Nickel-based HVOF coatings promoting high 562
temperature corrosion resistance of biomass-fired power plant boilers. Fuel Process. Technol. 563
2014; 125: 236-245. 564
[35] Uusitalo M A, Vuoristo P M J, Mäntylä T A. High temperature corrosion of coatings and 565
boiler steels below chlorine-containing salt deposits. Corros. Sci. 2004; 46: 1311-1331. 566
37
[36] Wu H, Bashir M S, Jensen P A, Sander B, Glarborg P. Impact of coal fly ash addition on 567
ash transformation and deposition in a full-scale wood suspension-firing boiler. Fuel 2013; 113: 568
632-643. 569
[37] Zheng Y, Jensen P A, Jensen A D. A kinetic study of gaseous potassium capture by coal 570
minerals in a high temperature fixed-bed reactor. Fuel 2008; 87: 3304-3312. 571
[38] Liu Y, Duan X, Cao X, Che D, Liu K. Experimental study on adsorption of potassium vapor 572
in flue gas by coal ash. Powder Technol. 2017; 318: 170-176. 573
[39] Wang G, Jensen P A, Wu H, Frandsen F J, Sander B, Glarborg P. Potassium Capture by 574
Kaolin, Part 2: K2CO3, KCl and K2SO4. Energy Fuels 2018; 32: 3566-3578. 575
[40] Punjak W A, Uberoi M, Shadman F. High-temperature adsorption of alkali vapors on solid 576
sorbents. AlChE J. 1989; 35: 1186-1194. 577
[41] Punjak W A, Shadman F. Aluminosilicate sorbents for control of alkali vapors during coal 578
combustion and gasification. Energy Fuels 1988; 2: 702-708. 579
[42] Tran K-Q, Iisa K, Steenari B-M, Lindqvist O. A kinetic study of gaseous alkali capture by 580
kaolin in the fixed bed reactor equipped with an alkali detector. Fuel 2005; 84: 169-175. 581
[43] Aho M, Ferrer E. Importance of coal ash composition in protecting the boiler against 582
chlorine deposition during combustion of chlorine-rich biomass. Fuel 2005; 84: 201-212. 583
[44] Wang G, Jensen P A, Wu H, Frandsen F J, Laxminarayan Y, Sander B, Glarborg P. KOH 584
capture by coal fly ash. Fuel 2019; 242: 828-836. 585
[45] Izquierdo M, Querol X. Leaching behaviour of elements from coal combustion fly ash: An 586
overview. International Journal of Coal Geology 2012; 94: 54-66. 587
[46] Kim A G, Kazonich G, Dahlberg M. Relative Solubility of Cations in Class F Fly Ash. 588
Environmental Science & Technology 2003; 37: 4507-4511. 589
38
[47] Blissett R S, Rowson N A. A review of the multi-component utilisation of coal fly ash. Fuel 590
2012; 97: 1-23. 591
[48] Vassilev S V, Vassileva C G. A new approach for the classification of coal fly ashes based 592
on their origin, composition, properties, and behaviour. Fuel 2007; 86: 1490-1512. 593
[49] Bashir M S, Jensen P A, Frandsen F J, Wedel S, Dam-johansen K, Wadenback J. 594
Suspension-Firing of Biomass. Part 2: Boiler Measurements of Ash Deposit Shedding. Energy 595
Fuels 2012. 596
[50] Bashir M S, Jensen P A, Frandsen F J, Wedel S, Dam-johansen K, Wadenback J, Pedersen 597
S T. Suspension-Firing of Biomass . Part 1 : Full-Scale Measurements of Ash Deposit Build-up. 598
Energy Fuels 2012; 26: 2317-2330. 599
[51] Wu H, Bashir M S, Jensen P A. Full-scale ash deposition measurements at Avedøre Power 600
Plant unit 2 during suspension-firing of wood with and without coal ash addition, Technical 601
University of Denmark, Denmark, 2012. 602
603
39